Soil suction and cracking from the onset to the end of desaturation: Micro-scale evidence and model
A multi-scale, multi-physics sequence of processes is discussed as developing during drying of non-clayey soils. Two variables are believed to be central in drying: suction resulting from the evaporation and effective stress associated with external constraints imposed on drying shrinkage. These two variables are tracked across the scales, both in experiments and simulations. The effective stress is critical as leading eventually to soil drying-cracking. Cracking is a most unwanted development in soil undergoing dewatering. Drying cracks often arise in the apparent absence of external forces. Hence, a tensile eigenstress pattern resulting from stiff inclusions, or tensile stress produced by reaction forces at the boundary constraints need to be contemplated to reach cracking criteria. An earlier tubular micro-scale model of porous drying medium indicates that transport of water toward evaporating surface during saturation phase induces a high suction. A critical suction value is reached at which water body boundary is penetrated in an unstable manner by air. At the meso-scale such air penetration constitutes a surface imperfection, inducing a total stress concentration near its tip, and in the presence of significant pore suction, a rapid increase in local tensile effective stress and crack propagation. Recent experimental results from a configuration of a cluster of grains provide geometrical data suggesting that an imperfection resulting from of air entry penetrates deep into the granular medium over 4-8 radii of the typical pore. Further evolution entails separation of grain clusters by funicular bridge instabilities, and in the last stage formation of two-grain pendular capillary bridges. The final phase is associated with a gradual decrease of the micro-scale suction within these elementary bridges, which eventually evolve into a positive pressure before the bridge rupture.
